This invention relates to a binder composition used for an electrode for a lithium ion secondary battery, a slurry of the binder composition, an electrode made from the slurry, and a lithium ion secondary battery fabricated by using the electrode.
In recent years, portable electronic appliances such as a notebook-sized personal computer, a portable telephone and a personal digital assistant have spread wide. Among secondary batteries used as a power source of the portable electronic appliances, lithium ion secondary batteries (hereinafter referred to as xe2x80x9cbatteriesxe2x80x9d when appropriate) are widely used. Portable appliances have been rapidly rendered small in size, thin in thickness, light in weight and high in performance in order to enhance the comfortableness as portable appliances. Thus portable appliances are utilized in many fields. With the broadening of use thereof, requirements for rendering small in size, thin in thickness, light in weight and high in performance are becoming severe also for batteries, similarly to portable electronic appliances.
To meet the above-mentioned requirements. attempts are being made for improving electrodes, electrolytes and other battery parts. As for electrodes, an active material and a collector, and further a binder for adhering an active material to a collector are being investigated. Generally a binder is incorporated in water or an organic liquid to prepare a binder composition, and in turn, the binder composition is mixed with an active material and, if desired, an electrically conductive material to prepare a slurry. A collector is coated with the slurry and the coating is dried to give an electrode. As for the binder composition, various polymers have been proposed as a binder ingredient to be contained in the binder composition.
Recently it is eagerly desired to develop a binder composition suitable for making an electrode of a secondary battery having a high capacity and exhibiting a minimized capacity reduction at repetition of a charge-discharge cycle.
For example, it is described in Japanese Unexamined Patent Publication No. H8-287915 that an electrode made by using as a binder a copolymer of an acrylic acid ester or a methacrylic acid ester, acrylonitrile and a vinyl monomer having an acid ingredient provides a positive electrode and/or a negative electrode of a lithium ion secondary batter having a high capacity and exhibiting improved characteristics at repetition of a charge-discharge cycle.
However, researches of the present inventors revealed that the lithium ion secondary battery made by using the copolymer binder disclosed in the above-mentioned patent publication still does not give a satisfactory characteristics at repetition of a charge-discharge cycle, that is, the initial capacity is on a satisfactory level but the capacity is reduced to a great extent at repetition of a charge-discharge cycle.
In view of the fore going, an object of the present invention is to provide a binder composition suitable for manufacturing a lithium ion secondary battery having a high capacity and exhibiting improved characteristics at repetition of a charge-discharge cycle. The present inventors made electrodes using various polymer binders to examine the influence of the polymer binders upon the characteristics of a lithium ion secondary battery, and found that, when a polymer of an ester of acrylic acid or methacrylic acid with ethylene glycol or polyethylene glycol is used as a binder for making an electrode, a lithium ion secondary battery with the electrode has a high capacity and exhibits totally improved characteristics at repetition of a charge-discharge cycle, including characteristics at repetition of a high-rate charge-discharge cycle and at repetition of a low-temperature charge-discharge cycle. The present invention has been completed on the basis of this finding.
Thus, in a first aspect of the present invention, there is provided a binder composition for an electrode for a lithium-ion secondary battery, comprising a polymer and a liquid dispersion medium; characterized in that said polymer comprises structural units represented by the following general formula (1) and is dispersed in the liquid dispersion medium: 
wherein R1 is hydrogen, a straight chain or branched alkyl group having 1 to 4 carbon atoms, or halogen, R2 is a straight chain, branched or cyclic alkyl group having 1 to 20 carbon atoms, or an aryl group having 6 to 30 carbon atoms, R3 and R4 are hydrogen or a methyl group, provided that R3 and R4 are not simultaneously a methyl group, and n is an integer of 1 to 50.
In a second aspect of the present invention, there is provided a slurry for an electrode for a lithium-ion secondary battery, characterized by comprising the binder composition according to the first aspect of the present invention and an active material.
In a third aspect of the present invention, there is provided an electrode for a lithium-ion secondary battery, characterized as being made by coating a collector with the slurry according to the second aspect of the present invention, and then drying a coating of the slurry.
In a fourth aspect of the present invention, there is provided a lithium-ion secondary battery, characterized in that one of a positive electrode and a negative electrode of the battery is the electrode according to the third aspect of the present invention.